1. Field of the Invention
The invention relates generally to optical receivers and more particularly to an optical receiver having signal level transient compensation.
2. Description of the Background Art
Optical transmission systems include a transmitter for transmitting a modulated optical signal into a link and a receiver for receiving the signal from the link. The link may span a short distance or thousands of kilometers. Existing optical receivers have an optical to electrical (OE) converter, typically using a photodiode, for converting the incoming optical signal to an electrical signal. The electrical signal drives a clock data recovery (CDR) system that recovers the clock and then uses the clock for sampling the electrical signal. The modulated data is recovered by comparing the sampled signal to a fixed decision threshold level. A sampled signal level that is above the threshold level yields a bit sense of one and a level below the threshold level yields a bit sense of zero.
When the decision threshold level is fixed, any variation or transient in average power of the incoming signal can lead to bit errors. Some workers have attempted to resolve this problem by leveling the variations and transients with optical devices in the link. This has been done with variable gain erbium-doped fiber amplifiers (EDFA)s and variable optical attenuators (VOA)s. However, these optical devices can be expensive for new systems and the installation cost for retrofitting into existing systems can be prohibitive.
There is a need for an inexpensive way to minimize bit errors when variations and transients occur in optical signal power.